1. Field of the Invention
This invention relates to printing systems using multiple scan beams and particularly to optical systems associated with acousto-optic modulators in such systems.
2. Description of Related Art
Printing systems including scanners are suitable for a variety of applications including printing text on paper, patterning photoresist during integrated circuit manufacture, and creating masks or reticules for projection-type photolithography systems. For integrated circuit applications, the printing systems typically require submicron precision. FIG. 1A illustrates the basic architecture of a precision printing systems 100 that employs scanning. System 100 includes: a light source 110 such as a laser; an acousto-optic modulator 120 that controls intensity of one or more input beams 135; prescan optics 130 that control the position, shape, and collimation of input beams 135; a scanning element 140 such as a polygon mirror that sweeps scan beams 145 along a scan direction; and post-scan optics 150 that focus scan beams 145 on an image plane 160. Scanning of scan beams 145 forms scan lines that expose a pattern in an image area of plane 160. Acousto-optic modulator 120 modulates the intensity of input beams 135 to select the pattern that scan beams 145 expose in image plane 160.
A conventional acousto-optic modulator includes a block of material such as fused silica through which input beams propagate. To turn on, turn off, or change the intensity of an input beam, a transducer generates an acoustic wave that crosses the path of the input beam in the block. The acoustic wave locally changes the optical properties of the block and deflects part of the input beam. A beam stop later in the optical train blocks the undeflected part of the beam. A concern for a precision scanner having a conventional acousto-optic modulator is the orientation of the scanning direction relative to propagation of the acoustic waves that modulate the input beams. If the propagation direction and the scanning direction are not collinear, turning beams on or off can reduce sharpness of edges or create undesired skew or directional bias in a pattern being illuminated. FIG. 1B illustrates an illuminated region 170 of a scan line formed when an acoustic wave deflects an input beam in a direction 178 (after convolution through the system optics 130 and 150) that is perpendicular to a scan direction 172. Deflection direction 178 typically corresponds to the direction of propagation of the acoustic wave. As acousto-optic modulator 120 turns on input beam 135, a cross-section 174 of the beam expands in direction 178. Accordingly, the initially illuminated part of region 170 is narrow and toward one edge until the input beam has a fully illuminated cross-section such as cross-section 175. Similarly, when acousto-optic modulator 120 turns off input beam 135, one edge of the input beam darkens first, and a shrinking cross-section 176 of the beam causes illuminated region 170 to recede toward the opposite edge. This reduces sharpness at the edges of illuminated regions formed by multiple scan lines, skews rectangular illuminated areas, and causes pattern lines at 45.degree. to the scan direction to differ in thickness from pattern lines at 135.degree. to the scan direction.
Acoustic waves in an acousto-optic modulator propagating opposite the scan direction (after convolution through scanner optics) eliminates skew and 45.degree./135.degree. bias and sharpens edges of illuminated regions. However, in scanning systems using multiple beams, projections of the scan beams along the scan direction typically overlap. For example, as shown in Fig. 1C, beams 132, 134, 136, and 138 overlap when viewed along scan direction 172. This creates a brush that illuminates a strip in the image plane without gaps between adjacent beams. With this configuration, an acoustic wave propagating along or opposite scan direction 172 would affect multiple beams. Generally, the separation 133 between beams inside acousto-optic modulator 120 must be more than a beam diameter to permit acoustic waves 122, 124, 126, and 128 to independently modulate respective beams 132, 134, 136, and 138. Accordingly, to provide independent control of the intensities of beams 132, 134, 136, and 138, acoustic waves 122, 124, 126, and 128 in acousto-optic modulator 120 must propagate at an angle relative to scan direction 172.
Systems and methods are sought that use simultaneous scan beams for faster scanning but avoid the skew, blurred edges, and directional bias associated with acousto-optic modulators having acoustic waves propagating at an angle to the scan direction.